Mirroring stopped working entirely after hours of using it without error. Blender 2.90
In edit mode (TAB), select all vertices (A). In mesh menu, click &quot;Symmetrize&quot; to fix the symmetry (use -X to +X or +X to -X). It should be fixed if you use X as mirroring axis.
While all vertices are selected, you should also fix your normals with Shift + N.
You should also try mirror modifier (but you have to delete 1 side).